Not Applicable.
Not Applicable.
1. Fielf of Invention
The invention relates to an arrangement for the manufacture of plastic molded parts such as sheets, profiles or the like, with at least two containers holding a liquid monomeric component which via feed lines and a mixing arrangement are connected to a mold in which the liquid plastic material solidifies into a molded article.
2. Description of Related Prior Art
It is already known to manufacture molded plastic articles by mixing two monomeric components situated in separate vessels, in particular caprolactam melts, to which an activator or catalyst is added, and which are then heated to an appropriate temperature. In that case, the monomeric components are cast in liquid state into appropriately tempered molds where they are polymerized without pressure. A disadvantage of this known method is that the molded articles are not manufactured continuously, but that the dimensions are limited by the size of the molds into which the liquid monomeric components are cast. Since such molds cannot be made available in a large enough number of different dimensions, the molded articles thus manufactured must be subsequently treated, in particular they must be cut to the desired size, which results in wastes that cannot be utilized. Furthermore, this known method as a rule leads to the formation of so-called feeder heads on the upper surfaces, similar to those formed in metal casting, which must be separated from the molded article after polymerization, which also requires an additional working process and leads to further waste.
Object of the present invention is to create an arrangement for the manufacture of molded articles which allows the continuous manufacture and the simple adaptation to the desired dimensions of the molded articles to be made, so that the production of waste can be avoided, and the work required for manufacturing can be reduced and simplified. To achieve this objective, the invention recommends that the mold is provided with two revolving conveyor belts running parallel to each other and arranged at a distance of each other, and that a distributor means is provided that feeds the liquid plastic material into the intake gap between the conveyor belts. If, as in the known method, the molded articles are formed from two polymerized monomeric components, it is advantageous to provide at least four containers, so that it is possible to switch to one container pair while the other pair becomes empty, thus avoiding the necessity to interrupt the continuous manufacturing process. The distributor means is used to distribute the liquid plastic material evenly across the entire width of the intake gap and subsequently moved on across the two conveyor belts whose length is designed so that this plastic material solidifies and is polymerized while it is being moved, and the finished solidified molded article exits from the outlet gap of the two conveyor belts.
It is practical to provide the feed line, preferably in the area where it leads into the distributor means, with a casting valve so that the quantity of the liquid plastic material can be adjusted by regulating the valve opening time, thus adapting the quantity to whatever is in demand.
It is practical for the conveyor belts to be downwardly inclined from the distributor means, and advantageously the incline can be adjusted. This ensures that the entire width of the conveyor belts can be loaded by the distributor means, and the occurrence of skips due to an insufficient material supply can be avoided. When the incline is adjustable, it can be adapted in particular to the thickness of the molded articles to be manufactured, which corresponds to the distance between the two conveyor belts. To manufacture molded articles of different thickness, it is therefore only necessary to make the distance between the two conveyor belts adjustable, which can be achieved without difficulty by mounting the deflection rollers on adjustable trestles.
According to a preferred embodiment of the invention, a sealing member, for example made of silicone rubber, is provided in the area of the two longitudinal edges of at least one conveyor belt. This sealing member may, for example, be formed of a compressible elastic hollow section which, due to its elasticity, can adapt itself to distances of different size between the two conveyor belts. By changing the position of the sealing members on the conveyor belt, the width of molded element to be manufactured can be changed without difficulty. Thus, the arrangement according to the invention makes it possible and simple to vary the thickness as well as the width of the molded elements to be manufactured.
To allow a simple change in the position of the sealing members, these lie loosely on the conveyor belt, according to another characteristic of the invention, and they are fixed in their position thanks to the support straps abutting on their outsides, which are supported by guides fastened to the conveyor belt preferably in the form of rivets. To change the position of the sealing members, only the support straps must be exchanged. These support straps also serve the purpose of preventing the lateral deflection of the elastic sealing members and thus to ensure that an exact lateral delineation is created for the molded articles to be manufactured.
When the manufactured molded articles are sheet-shaped, the liquid plastic material is fed by the distribution means directly to the inlet gap between the conveyor belts, whereby the shape of the sheet-shaped molded articles is determined by the distance between the conveyor belts and the distance between the sealing members. However, the present invention also allows the manufacture of molded articles which are not sheet-shaped, i.e. for example of profiles with a round or angular cross section. To manufacture such profiles according to the invention, cavities corresponding to the profiles to be manufactured, preferably made of silicone rubber, can be arranged between the two conveyor belts, and the liquid plastic material is then fed into those cavities. These mold sections are divided, with one part supported on the upper conveyor belt and one part on the lower conveyor belt. Each mold part is continuous and revolves with one of the two conveyor belts, while a closed mold is created which forms the mold for the cavities. Again, the molded articles can be fixed against lateral displacement via the guides fastened to the conveyor belt.
It is practical for the distributor means to consist of a tube system emptying into the inlet gap. If in the manufacture of thick sheets, the inlet gap between the two conveyor belts is large enough, it is possible that the distributor means extends into the space between the two conveyor belts without touching the belts, in which case no other lateral delineation is necessary. However, when the inlet gap is small, it is not possible to let the distributor means formed by the tube system extend between the two conveyor belts without touching the conveyor belts. In that case, stationary sealing members, preferably of triangular profile, must be provided to act as lateral limitation for the inlet gap.
It is practical for the distributor means to be provided with measuring contacts for measuring the quantity of the liquid plastic material fed into the intake gap, whereby the measuring contacts are preferably actively connected to the casting valve. In that case, the supply of the liquid plastic material is regulated via the casting valve, depending on the amount of liquid plastic material fed into the intake gap, i.e. when the maximum amount is reached, the casting valve closes, and when the minimum amount is reached, the casting valve opens. With such a control, the conveyor belts are driven at a constant speed that is precisely adjusted to the desired dose. However, the casting valve may also be open all the time, so that a certain amount of the liquid plastic material is fed constantly into the intake gap, in which case the drive of the conveyor belts is controlled via the measuring contacts.
An appropriately high temperature is necessary to solidify or polymerize the plastic material that is supplied. Therefore, the invention calls for a heating means in the area of the intake gap, preferably an electric radiant heater, which ensures the required polymerization temperature. To maintain this temperature, another characteristic of the invention calls for the conveyor belts to be arranged in a chamber which is provided on one front face, in the area of the distributor means, with an intake opening and on the opposite front face with a discharge opening for the molded article(s). In this case, the heating means is preferably arranged outside the chamber, whereby the chamber wall adjacent to the heating means is provided with a window, for example of vitrified ceramics, that allows heat radiation to permeate. This eliminates the danger that the caprolactam vapor will explode, which could happen at high temperatures.
An even temperature is achieved if the two front faces of the chamber are provided with air openings which are connected to an air circulation line provided outside the chamber, and if the line is preferably connected to a fan.
To remove liquid plastic material that may leak in the area of the sealing members which form the lateral limit of the mold primarily formed by the conveyor belts, collector troughs to catch such leaking liquid plastic material are provided below the longitudinal edges of the bottom conveyor belt, whereby the troughs empty into a collector bin connected to the chamber.
To prevent the unwanted deflection of the conveyor belts during solidification or polymerization, and thus to ensure the even thickness of the molded articles to be manufactured, a preferred embodiment of the invention calls for the conveyor belts to be provided with a sliding support on plates, preferably made of stainless steel. These plates may be provided with air-permeable openings through which compressed air can be supplied which creates a cushion of air between the conveyor belt and the supporting plate, thus reducing friction losses.
Preferably, an extracting roller connected to the discharge opening of the chamber and a separating means movable across the feed direction of the extracting roller are provided. The extracting roller ensures the removal of the solidified molded articles coming out of the discharge gap of the conveyor belts, and the separating means automatically cuts these articles to the desired length.
It is important that the conveyor belts can withstand the high temperatures of up to 190 C which occur during polymerization, and also that the belts are completely level and are not raised at the joint, which would result in an uneven surface of the molded articles to be manufactured. For those reasons it is practical if the conveyor belts are made either of continuous textile or plastic belts which preferably are provided with a coating of polytetrafluorethylene or of steel belts butt-welded at the joints.
The arrangement according to the invention also allows the manufacture of sheet-shaped molded plastic articles, in particular of polyamide, whose surfaces are provided with a covering layer in the form of a metal foil or the like, which is applied in a single operation. Such plastic sheets provided with a covering layer made of a metal foil or the like are already known, but although they have many advantages, they have not been well-received because until now, the covering layer had to be applied to the plastic sheets in a press, in a separate operation involving an adhesive, a process that has been complicated and expensive. According to the invention, an unwinding means can be provided above the top conveyor belt and/or below the bottom conveyor belt for a metal belt, consisting for example of aluminum foil or steel foil, which adjoins the conveyor belt and moves with same and is thus carried along into the intake gap by the moving conveyor belt. When the liquid plastic material is fed into the intake gap, it combines with the plastic material during polymerization which takes place between the two conveyor belts. Especially when the plastic material consists of cast polyamide 6 or a PA rubber block polymer, an excellent bond is achieved between the plastic material and the covering layers.
The molded articles manufactured by means of the arrangement according to the invention can also be fiber-reinforced. For that purpose, the invention suggests that at least one of the containers which holds the monomer can be connected to a device for admixing reinforcement fibers such as glass fibers, carbon fibers and/or aramid fibers, and preferably provided with a mixing means which ensures that the introduced fibers and the monomer are well-mixed.